The present invention relates to a device for tightening a bone screw into bone material during orthopedic or traumatologic surgery to a torque value M.sub.o which is less, by a predetermined fixed factor, than the torque value at which the bone material irreversibly deforms.
In orthopedic and traumatologic surgery, particularly those in the case of fractures, bone fragments to be joined are often fixed together by means of bone screws or bone implants in conjunction with such screws. A bone screw should be tightened to such an extend that on the one hand the bone fragments are pressed together as tightly as possible and on the other hand the bone fragments, the screw and the respective threads in the bone and on the screw are not damaged. A screw should therefore be tightened until the tightening torque has reached an optimum value slightly below that at which the threads are stripped.
Current practice is for surgeons to tighten the screws by feel. There have already been tests carried out to determine the torque values occurring, in which the surgeons have used a screwdriver with a torque measuring transducer. The results of these tests have been published in the paper "Dosierung des Drehmoments beim Einsetzen von Knochenschrauben" ("Dosage of Torque for the Insertion of Bone Screws") by J. Cordey, W. Widmer, A. Rohner and S. M. Perren in 115 Zeitschrift fur Orthopadie und ihre Grenzgebiete ("Journal of Orthopaedics and Allied Fields"), pp. 601&602 (1977.)
For surgeons who do not yet have much experience it is difficult to tighten the screws by feel just enough for the torque obtained at the end of tightening to be approximately equal to the optimum value. The surgeons could naturally use a screwdriver which limits the tightening torque for the screws to a pre-set definite maximum value. This would not be practical, however, since the various torque values at which the threads are stripped depend on the individual features of the bone and vary considerably from case to case, from bone to bone in the same individual and from location to location in the same bone. Tests in which screws with a diameter of 4.5 mm were screwed into the tibiae of fifteen different human cadavers have shown that the torque value at which the threads are stripped vary between about 1 and 7 Newton meters. If the torque were limited to a fixed value, this value would therefore have to be such that the threads would not be stripped even in the case of the bones with the lowest strength. This would then mean that when screws are inserted into bones of greater strength the torque would be far below the optimum for that bone.